Integrated circuit technology has revolutionized various fields including computers, control systems, telecommunications, and imaging. For example, in the imaging field, CMOS image sensors have proved to be less expensive to manufacture relative to CCD imaging devices. Further, for certain applications, CMOS devices are superior in performance. For example, the signal processing logic necessary can be integrated alongside the imaging circuitry, thus allowing for a single integrated chip to form a complete stand-alone imaging device. CMOS image sensors are now available from a variety of manufacturers, including the assignee of the present invention, OmniVision Technologies, Inc.
The primary building block of an image formed by a CMOS image sensor is a pixel. The number, size and spacing of the pixels determine the resolution of the image generated by the imaging device. The pixels of a CMOS image sensor are semiconductor devices that transform incident light photons into current signals. The signal produced by each pixel is generally extremely small, but is related to the amount of incident light photons.
Optimally, the image sensor must operate in a myriad of light and movement conditions. The image sensor when used in a mobile phone application may be required to record images in low light conditions, since a flash is not readily available. In such a situation, the exposure time of the image sensor must be increased, much like for a conventional camera using photographic film. However, the increase in exposure time will give rise to blurring of the image if the sensor is moved or shaken.
Further, in other applications, such as digital still cameras, the dynamic range of the image sensor may be inadequate to compensate for an image that has both low light sections and high light sections. In other words, shadows and light are not adequately resolved. Dynamic range is generally defined as the highest possible unsaturated signal divided by the noise floor of the pixel. Thus, dynamic range is related to the signal-to-noise ratio (SNR).
These problems noted above are particularly endemic to CMOS image sensors, which have a lower light sensitivity and narrower dynamic range than charge coupled devices (CCD's).
Prior attempts to solve these problems have included increasing the exposure time using an electronic shutter. However, this method cannot be applied to pinned photodiodes and may result in movement artifacts.
Another method utilizes frame addition using multiple frames that are captured of the same image. However, this technique relies upon one frame for middle and low light levels and another frame for high light levels. This is accomplished using different exposure times for each frame. Nevertheless, it is difficult to add frames that have different exposure times and this degrades picture quality.
Another method uses an image sensor that has pixels with two different fill factors. This type of sensor though has poor resolution for low light level images. Finally, another method uses a high frame driving rate that includes analog-to-digital (ADC) circuit in each pixel. This causes a large pixel size. Further, resolution is limited since movement artifacts may appear.